A device for measurement of basic sediment and water (BS&W) in a predominantly non-aqueous stream (e.g. pipeline crude oil) is disclosed in U.S. Pat. No. 4,184,952. This device is an improvement on capacitance type instruments of the art which are dependent upon the extent to which the intrinsic dielectric constant of the subject fluid varies with time. The gravity and physical composition of crude oil are two factors which determine its intrinsic dielectric constant. If one or both of these properties should vary, instruments measure the accompanying change in the dielectric constant as percent BS&W. This yields an inaccurate measurement of BS&W because instruments must be initially set to read zero BS&W at the intrinsic dielectric constant of the fluid. The capacitance type instruments of the prior art have no means for automatically correcting the zero BS&W setting to compensate for periodic variations in the oil properties mentioned. By comparison, the device of U.S. Pat. No. 4,184,952 provides for automatic compensation of BS&W measurements by producing a clean, dry sample of the line fluid for measurement of its intrinsic dielectric constant. In this way, the true BS&W content of the fluid is measured by finding the difference between the dielectric constants of the wet and dry streams.
Even though the improvement over the prior art represented by the invention of U.S. Pat. No. 4,184,952 is substantial, it now has been discovered that other improvements can be made which even further improve the efficiency and accuracy of this invention. Thus, temperature variations within the apparatus of the invention can cause some loss of accuracy of readings. For example, the indications from the wet and dry cells of the apparatus are compared to determine BS&W content. This comparison presumes that all other properties of the oils are identical, including temperature. The error introduced by temperature difference is about 0.02% BS&W per degree Fahrenheit. While this error might be neglected if the temperature difference were only one or two degrees, it is desirable to reduce error introduced by a larger temperature difference, for example up to 18.degree. F. temperature difference between the wet and dry cells as have been noted with the apparatus of U.S. Pat. No. 4,184,952.
In addition, changing temperatures within the apparatus of U.S. Pat. No. 4,184,952 tend to cause wax to plate the inner surfaces of the various parts thereof. This will usually occur when the stream leaving the pipeline is cool enough to contain traces of solidified wax. Passage through the pump of the apparatus, centrifugal filter and associating tubing tends to raise the temperature of the oil and melt some of the crystals. The melted wax will then re-solidify on any surfaces that are cooler than the oil. This occurs, for example in a heat exchanger, to be described hereinafter, wherein the warm, dry oil leaving the centrifuge is cooled by the incoming wet stream from the pipeline. In this case, the wax is detrimental in two ways: (1) it tends to impede flow and (2) it reduces the heat exchange capacity of the affected surfaces. It also has been observed that a certain amount of residue tends to collect in the dry oil capacitance measurement cell. The cell comprises two coaxial tubes electrically insulated from each other within the annular space between the tubes, which constitutes an electrical capacitor whose value depends upon the material in the annular space. The residue buildup usually tends to cause an error in the direction of wetness. Thus, the dry cell indicates less dry than it should compared to the wet cell, where flow is more rapid and the residue does not tend to accumulate.
Accordingly, the present invention is directed to overcoming the above and other problems described hereinafter by means of the following disclosed improvements.